Heat gun

ABSTRACT

Aimable hand held shrink gun for plastic film producing heated air in 250* to 1000*F range employs high performance internal combustion burner discharging exhaust gas at velocity above 4000 feet per minute and temperature on order of stoichiometric burning temperature, i.e. 3,450*F for propane fuel. High velocity exhaust gases enter mixing zone preferably in a divergent manner and with perimeter of gas flow cross-section at least 25% greater than the perimeter of a circle of equal area, to provide an extended gas-air interface. The exhaust gas propels and heats a large volume of ambient air, producing in a practical small distance a useful flow of shrinking air at the desired temperature. Preferably the burner outlet is of elongated form with decreasing cross-section towards outlet, e.g. a multiple legged outlet cross-section. Preferably a positioning means set a minimum distance between work piece and outlet. Where the exhaust gas stream is exposed to admission of increasing air along the length, the length of the positioning means sets the working temperature. Preferably a shield extends about the exhaust gas stream, preferably in the form of a tube with space for ambient air. With apertures along the tube length, the mass of air increases and decreases rapidly in temperature therealong. With a closed wall tube its smallest cross-section defines the amount of ambient air entrained and thereby sets the discharge temperature. Burners useful in this heat gun are of high capacity type with jet pump feed, pressure recovery passage and flame holder positioned at entry of fuel mixture into the burner chamber. By combining an ignition chamber upstream of the burner outlet with a flameholder having thin blades and limited bluff-body effect, an increase in performance is achievable.

United States Patent [1 1 Zagoroff *NOV. 4, 1975 [5 1 HEAT GUN [76]Inventor: Dimiter S. Zagoroff, 4 Brigham ST.,

East Boston, Mass. 02128 Notice: The portion of the term of this patentsubsequent to Dec. 18, 1990, has been disclaimed.

[22] Filed: Apr. 16, 1973 [21] Appl. No.: 351,359

Related U.S. Application Data [63] Continuation-impart of Ser. No.197,207, Nov. 10,

1971, Pat. N0. 3,779,694.

Primary ExaminerJohn J. Camby [57] ABSTRACT Aimable hand held shrink gunfor plastic film producing heated air in 250 to 1000F range employs highperformance internal combustion burner discharging exhaust gas atvelocity above 4000 feet per minute and temperature on order ofstoichiometric burning temperature, i.e. 3,450F for propane fuel. Highvelocity exhaust gases enter mixing zone preferably in a divergentmanner and with perimeter of gas flow crosssection at least 25% greaterthan the perimeter of a circle of equal area, to provide an extendedgas-air interface. The exhaust gas propels and heats a large volume ofambient air, producing in a practical small distance a useful flow ofshrinking air at the desired temperature. Preferably the burner outletis of elongated form with decreasing cross-section towards outlet, e.g.a multiple legged outlet cross-section. Preferably a positioning meansset a minimum distance between work piece and outlet. Where the exhaustgas stream is exposed to admission of increasing air along the length,the length of the positioning means sets the working temperature.Preferably a shield extends about the exhaust gas stream, preferably inthe form of a tube with space for ambient air. With apertures along thetube length, the mass of air increases and decreases rapidly intemperature therealong. With a closed wall tube its smallestcross-section defines the amount of ambient air entrained and therebysets the discharge temperature. Burners useful in this heat gun are ofhigh capacity type with jet pump feed, pressure recovery passage andflame holder positioned at entry of fuel mixture into the burnerchamber. By combining an ignition chamber upstream of the burner outletwith a flame ,holder having thin blades and limited bluff-body effect,an increase in performance is achievable.

37 Claims, 11 Drawing Figures U.S. Patant Nov. 4, 1975 Sheet 1 of33,917,442

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U.S. Patent Nov. 4, 1975 Sheet 2 of3 3,917,442

US. Patent Nov. 4, 1975 Sheet 3 of3 3,917,442

h m %O...|,.O| O O 0 O O Q 0 0 0 0 0 0 0 E .III O O 3 on O b- HEAT GUNThis is a continuation-in-part of US. Pat. Application Ser. No. 197,207,filed Nov. 10, 1971, now US. Pat. No. 3,779,694.

Numerous applications in industry and home require low temperatureheating, in particular heating of plastic film to shrink it aboutvarious objects to form secure, water-proof covers about the objects,etc. Low temperature in the range of 250 about 1000F is extremelyimportant since higher temperatures lead to blistering, cracking andcharring of inherently low temperature materials. One tool used for thispurpose is the electric heat gun. An electric blower passes cold airover a resistance heating element, and the hot air is directed at thework piece. Two disadvantages are that the power (hence flow-rate ofheated air) is limited to 3 kw using common electric outlets rated for30 amp fuses and the tool is not usable in the field where electricityis not available.

To get around the first limitation, units have been built in which a gasflame supplies the heat and a blower is used to mix in tempering air.These units, incorporating two different power systems, are relativelycomplicated, bulky, and expensive. A typical 25 kw unit intended forhand held use weighs about 12 lbs.

Units that rely on fuel alone, such as hand held torches have theproblem that the flame temperature of common fuels such as natural gasor propane are quite high, above 3000F, many more times the desiredtemperature. In an attempt to avoid overheating the product, effortshave been made to slow down the flame being applied to the work piece bymeans such as spreaders, or by employing fuel rich, so-called yellowflames, but still hot spots, overheating, scorching and charringproblems persist.

According to the present invention, it is discovered that a much moresatisfactory low temperature heating device for use as a shrink gun canbe provided. A jet of fuel gas first entrains by jet pump action aquantity of atmospheric air for combustion, followed by recovery of thevelocity head to produce a pressure exceeding atmospheric, followed by acombustion chamber whose outlet directs high velocity, high temperatureair into a propelling zone. In the propelling zone the combustion gasespropel relatively larger quantities of ambient air in the same directionwith attendant moderate heating thereof. The invention employs anintentionally high velocity, intense burning action rather than thecommon low velocity, diffused burning pattern. This appears at firstsight contradictory. Commonly, higher velocity burners are employed toachieve faster, more intense heating rates. Such behavior can beillustrated by plotting the time needed to heat the end of e.g. a copperpiece held at the burner outlet, using the same energy input but varyingthe exhaust gas velocity directed against the copper. The higher thevelocity, the sooner the copper will be heated, and at high velocitiesthe copper will melt. The inference here is that when more gentleheating is sought, lower velocities should be employed. The prior artgas torches described above attempted to employ this principle toachieve gentle heating.

But according to the present invention, using the high velocity, intenseburning, the work piece is held some distance away from the burner,quite beyond the flame. A jet pump effect then is employed to entrain,propel 2 and mix with large quantities of ambient air. A high velocityburner according to the present invention can thus produce a large airflow of desired low temperature (consisting mainly of ambient air whichis propelled in the same direction and. heated by the combustion gases)within a short distance.

These good results are attributable, it is believed, to a combination offactors. The much higher velocity (for a burner of given fuelconsumption) leads to a smaller area outlet aperture, which leads to alarger ration of cross-section perimeter to cross-section area of thestream, which leads to a more effective air entrain ment interface, i.e.higher pumping rate and mixing rate for a given length of the mixingzone. This can be enhanced by flattening the burner outlet area orotherwise shaping to get an extended perimeter of gas-to-air interface.As more and more cold air is drawn in, the momenntum of the exhaust gasfrom the combustion chamber is spread over a much. greater air mass;however, the velocity at the work piece remains sufficiently high toachieve good heat transfer.

Furthermore the short time it takes for the exhaust gases according tothe present invention to reach desired low treating temperature meansthat they are not subject to detrimental buoyancy forces. Note that forslower burner output velocities, and slower cooling found in priordevices, the stream of gaseous products is exposed for a quite long timeto the effects of the general surroundings as it moves to the work pieceand it thus starts curving upwards due to buoyancy forces and becomesseriously prone to being deflected by drafts of air, becominguncontrollable.

According to the present invention, it is thus proposed to construct aheat gun with a high velocity burner and to entrain air with a jet ofthe high velocity exhaust gases to produce an air blast of intermediateor low temperature. In one preferred embodiment the air entrainment cantake place as in a free jet with a predetermined distance between burnerand work piece. The pumping or entrainment zone preferably is within anopen metal cage, one function of which is to space the burnerpredictably from the work piece to assure a predetermined blasttemperature delivered to the work piece and another function is theadmission of additional air along the length. The peak gas producttemperature is a predictable function of spacing to the work piece, thusan adjustable cage or other standoff or positioning means can be presentand precalibrated for different output temperature requirements.

Another preferred embodiment employs a closed mixing tube of bigger (byat least 5 times) cross-sectional area than the burner outlet area.Given a sufficient length of mixing tube, generally 3 to 7 diameters, orshorter where highly dispersive burner outlets are used, thisconstruction assures complete mixing of the burner products with theentrained air resulting in high uniformity in temperature of theresulting stream. The degree of temperature attenuation (or mixingratio) is here governed by the ratio of the mixing tube and burneroutlet cross-sectional area, and thus a desired temperature can bereproduced repeatedly by the predetermined sizing.

In preferred embodiments the outlet of the combustion chamber is shapedsuch that the outlet cross-sectional area and the downstreamcross-section assumes a shape with a perimeter substantially greaterthan the radius of a single circle of the same area. Such outletstypically take the shape of slits, or multiple rounds. By

this means the mixing length to achieve a desired temperatureattentuation is reduced in direct proportion to the ambient-to-exhaustgas (jet pump) interface, defined by the exposed perimeter of the streamcross-section. This behaviour can be illustrated by comparing the mixinglength of two burners having the same fuelbuming capacity and sameexhaust gas velocity but different combustionchamber outletconfigurations. An outlet that has at least percent more flow perimeterthan a round outlet achieves a desired temperature such as 600F in 12inches, contrasted with percent more length for the circular outlet.

Further reduction in mixing length can be achieved if the combustionchamber is shaped such that the streamlines of the exhaust gases assumea divergent pattern from the centerline, so that the perimeter of thestream cross-section downstream of the outlet is greater than at theoutlet and to separate the exhaust gas molecules from each other as muchas possible to maximize exposure to and mixing with ambient air. In thecase of multiple outlets such a pattern can be achieved by inclining theaxes of the flow paths away from the center line. In the case of slits,such a pattern can be achieved by tapering the walls of the combustionchamber away from the center line but maintaining a constantlydecreasing cross-sectional area of the chamber in the region immediatelyupstream of the outlet to avoid diffusion or separation of the flowinside the chamber.

Accordingly to another feature, different outlet sections associatedwith different combustion chambers receive fuel-air mixture from acommon primary jet pump activated only by a jet of gaseous fuel.

A further feature of the invention which enhances the velocity of theexhaust gases from the combusion chamber-and hence the secondarypumpting effect on ambient air-also has independent significance ingeneral for burners powered by liquid gas fuel sources or otherpressure-limited devices. This feature comprises the use of bluff bodyflame holders of known types, and particularly those which swirl thefuel-air mixture entering the combustion chamber and produce eddyingeffects for stabilizing the combustion process in the wakes of theblades. In such prior constructions the trailing portions have beenintentionally very blunt, for producing a so-called bluff body effect,enabling significant recirculation of portions of the gases in thecombustion chamber. While such bluff bodies have enabled successfulcombustion, it is realized that they introduce a significant pressureloss, so that much of the pressure in the upstream recovery passagefails to be usefully converted to velocity of the gas. According to afeature of the invention, it was hypothesized that much of therecirculation heretofore felt necessary was really only necessary duringthe initial stages of ignition, for carrying the flame back from theoutlet to the inlet of the combustion chamber. According to theinvention an igniting recess is provided adjacent the flame holder,upstream of the combustion chamber outlet. Also, the ignition recess ispositioned out of the mainstream of gases flowing from the flame holdermeans but in communication therewith, this recess being provided with aspark means for initially igniting the gas fuel mixture. With thisconstruction smaller or thinner bluff bodies with smaller pressure dropcan be employed, with reduced bluff body effect. Such flame holders haveinsufficient eddy effects to produce recirculation of gas from theoutlet of the combustion cham- 4 ber, but sufficient to producerecirculation of gas from the vicinity of the ignition recess. It isfound that such a construction successfully initiates as well assustains combustion, while the pressure loss across the flame holder isgreatly reduced, and a corresponding increase in gas velocity effectscan be achieved.

FIG. 1 is a partially diagrammatic verical cross-sectional view of apreferred embodiment having a shrouded propelling and mixing zone and aflattened and diverging burner outlet. FIGS. la, 2 and 3 are transverseviews taken on lines 1a, 2 and 3 respectively in FIG. 1.

FIG. 4 is a view similar to FIG. 1 of a second preferred embodiment;FIGS. 5 and 6 are transverse views taken on lines 5 and 6 respectivelyof FIG. 4;

FIG. 7 illustrates a pallet carrying an object about which is shrunk aplastic film bay by use of a shrink gun according to the invention.

FIG. 8 is a view similar to FIG. 1 of another preferred embodiment andFIGS. 9 and 10 are transverse views taken on the respective lines ofFIG. 8.

Referring to FIGS. 1-3 and FIG. 7 pressurized gas G passes throughnozzle 1. The nozzle aims into a duct 3. The nozzle-duct combination iscommonly known as a jet pump and its function is to entrain atmosphericair A from openings 0 around the nozzle, between struts 2, see FIG. 1a.The duct comprises a first inlet section of gradually converging form3a, then a straight section 3b followed by divergent section 30 and thena short length of straight, pressure recovery section 3d. The pumpformed by the inlet, and subsequent straight, divergent and straightsections provides a fuel-air mixture in section 3d at as high a pressureas possible, typically 2 inches water column, up to 4 inches watercolumn, assuming a pumping pressure of 20 psi for gas G, as from propanesource S.

The mixture is directed into the burner. The burner consists of aninternal combustion chamber 5 and the bluff body flameholder 8. Gas isburned in the combustion chamber. Flame is prevented from flashing backinto the jet pump because of the design of the flameholder. Passages,dimension e, are so small that the gas velocity therethrough is greaterthan the burning velocity so the flame simply cannot travel upstream. I

At cross-sectional view FIG. 2 the combustion chamher is cylindrical,and then flattens out, FIG. 3. In this embodiment, with a spreading flowof the exhaust gases it is important that in the latter part of theburner, after combustion has occurred, the passage has equal, or asshown, decreasing cross-sectional area while, to extend the wettedperimeter, it fans out in one direction. The flow cross-section areaupstream is larger than the outlet 5 FIG. 3. The gases are thusaccelerated as they come out of the burner. The geometry is particularlyimportant in this latter half of the burner, to maintain velocity andavoid separation of the stream from' the diverging walls.

In this particular burner embodiment there is first a cylindricalsection 5a less than one diameter in length from the flameholder andthen a transition section 5b. In operation combustion initiates at theflameholder and spreads downstream.

The air enters the inlet 3a at a slow velocity and speeds up to a veryhigh velocity inside pump 3 reaching, in a typical embodiment, a maximumaround 8,800 fpm in section 3b. In the diffuser 30 it slows, thevelocity energy converting to static pressure head, reaching a velocityof 2,200 fpm in pressure recovery section 3d.

When the gas enters the burner 5 and is heated it tends to expand and itincreases in velocity again to a maximum in the outlet 5 of the burneraround 14,570 fpm. From then on the gas starts to entrain largequantities of ambient air A and the mixture slows down.

In a typical embodiment according to FIG. -1, the following conditionscan be measured:

In this embodiment the mixing process is obtained in what is called afree jet in which air can enter the mixing stream at any pointdownstream, entering through the series of apertures 19 distributedalong the length of tube or shroud 11. This mixing is verylengthdependent. As can be seen from the foregoing table the furtherdownstream from the burner the more air is drawn in, the lower thetemperature has dropped. This temperature attenuation curve is verypredictable for each size of outlet and velocity through it. Due to thefact that there is an ever decreasing temperature, one can select thetemperature wanted. To assure constant spacing, a device such as cage 11serves to position the burner relative to the workpiece and still admitair for mixing.

In the embodiment of FIG. 1 for different temperatures at the end ofshroud l 1 one needs only to position its end in the appropriatelocation. It thus sets a maximum temperature deliverable to theworkpiece and by moving away one can obtain lower temperatures.

The embodiment of FIG. 1 has other details. At the entry to thecombustion chamber the flame holder 8, as seen in FIG. 2, has aplurality of helical blades set about a center, with thin trailingedges 1. Adjacent this flame holder is a recess 13 out of the mainstreambut in communication therewith. In this recess is spark plug 15 which isactuated by a hammer blow upon piezo crystal 17 located in the handle,actuated by plunger 21.

This unit can readily be made to have for instance a rating of 30 kw100,000 BTU per hour) with outlet velocities for instance of 1500ft/min, at output temperatures of 1000F, or with suitable shortening orlengthening of member 11, respectively higher and lower velocities andtemperatures.

The unit is light-weight and can be readily aimed at a plastic film bag20 of 5 feet dimension on each side, see FIG. 7 pulled over an articleon a pallet. By holding the unit of FIG. 1 one or two feet away from thefilm, and progressively sweeping the output across, up and down andaround the object, the plastic film (e. g. plyethylene of 0.006 inchthickness) rapidly shrinks until it tightly hugs the article, providinga weather proof covering therefor.

The embodiment of FIGS. 4-6 is identical to that of FIG. 1 up to line L.From there the gas passage 6 branches to a number of small combustionchambers 30 each of which has its own helically bladed flame holder 35having thin trailing edges. One of the combustion chambers has anignition recess as described for FIG. 1, and another has a passageway 37communicating with the first, to be ignited thereby.

The outlet sections of each combustion chamber diverge as describedabove, terminating in an elongated outlet slit, these slits being in anarray with axes X and Y of the long dimensions intersecting at thecenter of the array. These individual outlets and the array areeffective to produce a very large jettoair interface for effectivepumping and entrainment of air, as in duct 7.

The duct 7a is open at its ends and has imperforate walls along itslength.

In effect the hot gases from the burners 30 drive a second jet pump, mixand heat ambient air A. Duct 7 of this second jet pump has a crosssection area, shown in FIG. 6, which is substantially larger than theoutlet area of the burner, with an order of magnitude from 5 to 50. Theair inlet 7a to duct 7 is of corresponding size, due to its flare form,positioned by struts 6 concentrically about the burner. The velocity ofthe hot gases entrains cold air, and this stream mixes in the duct, thefunction of the duct being to equalize the velocities and thetemperature of the mixture. If the duct were cut too short a hot coreand a cold outside would be found. Complete mixing occurs so that aftera length of more than about 3 diameters up to 7 depending upon design,equal temperature and equal velocity come out. The amount of airentrained is governed primarily by the area ration of duct 7 to theburner outlet area. Typically, to achieve the same low temperature withsame burner design, the length of shroud 11 of FIG. 1 beyond the burnerwill be less than the length of tube 7 of FIG. 14, and hence FIG. 1 maybe more convenient for certain applications. Where the uniformity of thetemperature of all air emitted from the outlet is important, one maychoose however the embodiment of FIG. 4 over that of FIG. 1. Anotheradvantage of FIG. 4 is that it is windproof in high cross winds, usefulfor instance around airports and railroads.

Referring to the embodiment of FIGS. 8-10, it is identical in principleto FIG. 4 up to flameholders 35, the exception being that the end of thesingle pressure recovery passage discharges into twin branches passagesand combustion chambers instead of into four of them. In this particularembodiment the slit-form outlets of the combustion chambers are spacedapart with their long dimensions parallel, and side by side. The pair ofoutlets discharge into a perforated shroud 1 1b, similar in itsapertures to the shroud of FIG. 1, but being of generally squarecross-section, see FIG. 10. In a preferred embodiment the shroud is 6inches long measured from the outlet of the combustion chamber and 8inches wide and high. In an embodiment identical to FIG. 1 up to line L,at its outlet T there is observable a temperature of 750F, peak velocityof 1340 fpm and a mass flow rate of l 1.6 pounds per minute. Thiscompares most closely to the 12 inch distance for the embodiment of FIG.1 and thus demonstrates that the same output can be got over muchshorter propelling and mixing distances using multiple outletarrangements.

In typical use, in shrinking plastic pallet wrap about objects onpallets, the operator will take into account how fast he moves the gunand the thickness and nature of the plastic film when determining theparticular distance he employs between the gun and the thermoplasticfilm. For instance using a 0.006 thick polyethylene film and a shrinkgun according to FIG. 1 rated at 120,000 BTU per hour a film section of1 foot height can be shrunk at a speed of travel of the gun of 32 feetper minute, at a spacing of 1 foot away, measured from the combustoroutlet. Also, where tears in the film occur, he may patch them simply bymoving the gun closer, softening the area of the tear and then applyinga patching piece of similar thermoplastic film.

It will be understood that numerous variations in the specificconstruction are possible within the spirit and scope of the followingclaims.

I claim:

1. A hand held aimable shrink gun plastic film capable of providing ashrink-producing flow of heated air in the 250F to l000F range againstplastic film lying over an object to be covered, the shrink gun relyingupon fuel along without assistance of blowers or compressors, saidshrink gun comprising a gas jet adapted for connection to a conventionalfuel gas source such as propane having a stoichiometric burningtemperature substantially exceeding 3000F, a jet pump activated by saidgas jet and having an opening for drawing atmospheric air for combustioninto a subatmospheric pressure region produced by said jet, said jetpump constructed to impart velocity to said combustion air by mixing, anenlarged pressure recovery passage into which the mixture of gaseousfuel and combustion air proceeds, said recovery passage constructed toconvert velocity head of said gases to a pressure head exceedingatmospheric pressure, an internal combustion chamber having an entryinto which said pressure recovery passage discharges, said internalcombustion chamber having a flame holding means at said entry and anoutlet discharging into an ambient air propelling and mixing zonepreceding said work object, the respective parts of said shrink gunconstructed to introduce and burn said fuel in substantiallystoichiometric conditions and discharge combustion gases into saidpropelling zone at a temperature exceeding 3000F and a velocity inexcess of 4000 feet per minute in a manner to propel relatively largerquantities of ambient air in the same direction with attendant heatingthereof by said combustion gases, thereby to produce a flow against saidplastic film at temperature in the 250F to 1000F range, consisting inmajor part of ambient air propelled and heated by said combustion gases.

2. The shrink gun of claim 1 including positioning means setting aminimum length of said propelling and mixing zone between said workpiece and combustion chamber outlet, ensuring a flow of heated air belowa predetermined maximum temperature upon the work piece.

3. the shrink gun of claim 2 wherein the combustion gases are exposedduring their travel through said propelling and mixing zone to admissionof increasing quantities of air.

4. The shrink gun of claim 2 wherein said positioning means comprises ashield member extending at least partly about the stream of combustiongases, said member providing air flow space whereby ambient air can flowfrom surroundings into contact with said combustion gases.

5. The shrink gun of claim 4 wherein said shield member has openingsalong its length exposing the traveling gases to admission of additionalair therealong.

6. The shrink gun of claim 1 wherein said combustion chamber outlet hasa cross-section perimeter greater by at least 25% than the perimeter ofa single circle of identical cross-sectional area.

7. The shrink gun of claim 6 wherein said combustion chamber has aconstant or decreasing flow cross-section area leading to said outlet.

8. The shrink gun of claim 7 wherein said outlet comprises an elongatedoutlet aperture, said combustion chamber having walls diverging in thedirection of elongation of said aperture.

9. The shrink gun of claim 6 wherein the outlet crosssection comprises amultiplicity of elongated slits.

10. The shrink gun of claim 1 wherein said outlet has portions directedin divergent outward directions.

11. The shrink gun of claim 10 wherein said outlet comprises amultiplicity of outlet apertures, axes of some of said aperturesdiverging relative to other of said apertures.

12. The shrink gun of claim 1 wherein said combustion chamber has arestricted entry connected to said recovery passage, said entry being ofsubstantially smaller cross-section than said passage and than thecombustion chamber downstream thereof, said entry adapted to produce aninlet velocity to said combustion chamber greater than the flamevelocity of said gas.

13. The shrink gun of claim 12 including a tubular member surroundinngthe outlet of said combustion chamber, said tubular member providing anair inlet flow cross-section in the vincity of said outlet which isgreater than 5 times the flow area of said combustion chamber outlet,said tubular member having a length longer than said combustion chamber.

14. The shrink gun of claim 13 wherein said tubular member has air inletopenings along its length, for adding air to the stream flowing throughsaid tubular member.

15. The shrink gun of claim 13 constructed to produce a predeterminedoutlet temperature wherein walls along the length of said tubular memberare imperforate, the inlet for ambient air to said tubular memberpositioned in the vicinity of said combustion chamber outlet, and beingin the range of 5 to 50 times the volume of combustion gases, saidoutlet and said tubular member cooperating to provide a jet pump forambient air.

16. The shrink gun of claim 1 wherein said combustion chamber has anexhaust outlet flow cross-section of extended perimeter, preferably theoutlet formed as an elongated slit, said outlet defining a propellingmeans having an extended jet-air interface effective to propel and mixwith large quantities of ambient air over a short distance beforereaching said object to be heated.

17. The shrink gun of claim 16 wherein said combustion chamber in theregion immediately preceding said outlet has walls arranged so that theflow cross-section does not increase leading to said outlet, said wallscooperating to achieve a high exit velocity at said outlet forpropelling said ambient air.

18. The shrink gun of claim 17 wherein said flow cross-section of saidcombustion chamber decreases leading toward said outlet while one pairof walls diverge from each other in the direction of said outlet toproduce a high velocity, divergent jet having a progressively enlargingjet-air pumping interface, for increased entrainment of air.

19. The shrink gun of claim 16 wherein there are a plurality of outletsections for said high velocity gas,

9 preferably these outlet sections being set at angles to one another,providing a large jet-air pumping interface for increased entrainment ofair.

20. The shrink gun according to claim 19 wherein a single combustionchamber has angled walls defining gradually diverging streams, the endsof the walls defining said plurality of combustion chamber outletsections.

21. The shrink gun of claim 19 wherein certain outlet sections areassociated with different combustion chambers, all of said combustionchambers receiving fuel-air mixture from a common mixing chambercomprising a primary jet pump activated only by a jet of gaseous fuel.

22. The shrink gun of claim 1 having a bluff body flame holder means inthe path of air-fuel mixture entering said combustion chamber, producingeddy effects for stabilizing the combustion process, known per se, andcharacterized in that said combustion chamber has an ignition recessadjacent said flame holder means and well upstream of the outlet of saidcombustion chamber, said ignition recess being out of the main stream ofgases flowing from said flame holder means but in communicationtherewith and having spark means known per se for initially igniting theair fuel mixture, and the said flame holder having insufficient eddyeffects to produce recirculation of gas from the outlet of saidcombustion chamber, but sufficient to produce recirculation of gas fromsaid ignition recess whereby ignition can be initated while said flameholder introduces only a small pressure loss to gas entering saidcombustion chamber.

23. The shrink gun according to claim 22 characterized in that saidbluff body flame holder comprises a series of helical blades (known perse), the end of said blades being tapered producing only a limited bluffbody effect.

24. A hand held aimable shrink gun for plastic film capable of providinga shrink-producing flow of heated air in the 250F to 1000F range againstplastic film lying over an object to be covered, the shrink gun relyingupon fuel alone without assistance of blowers or compressors, saidshrink gun comprising a gas jet adapated for connection to aconventional fuel gas source such as propane having a stoichiometricburning temperature substantially exceeding 3000F, a jet pump activatedby said gas jet and having an opening for drawing atmospheric air forcombustion into a subatmospheric pressure region produced by said jet,said jet pump constructed to impart velocity to said combustion air bymixing, an enlarged pressure recovery passage into which the mixture ofgaseous fuel and combustion air proceeds, said recovery passagecoonstructed to convert velocity head of said gases to a pressure headexceeding atmospheric pressure, a plurality of internal combustionchambers each having an entry into which said pressure recovery passagedischarges, each of said internal combustion chambers having a flameholding means at its said entry and an outlet discharging into anambient air propelling and mixing zone preceding said work object, therespective parts of said shrink gun constructed to introduce and burnsaid fuel in substantially stoichiometric conditions and dischargecombustion gases into said propelling zone at a temperature exceeding3000F and a velocity in excess of 4000 feet per minute in a manner topropel relatively larger quantities of ambient air in the same directionwith attendant heating thereof by said combustion 10 gases, the outletsof said plurality of combustion chambers combining to produce a flowagainst said plastic film at temperature in the 250F to 1000F range,consisting in major part of ambient air propelled and heated by saidcombustion gases.

25. In a burner for gaseous fuel activated only by the ambient pressureof a liquid gas fuel source, suitable for use in heat guns and torches,and comprising a mixing chamber directing air fuel mixture into acombustion chamber, the mixing chamber in the form of a primary jet pumpactivated only by a jet of fuel from said source which jet entrains airfrom the atmosphere and forms a fuel-air mixture, a pressure recoverypassage of increased cross-section wherein velocity head produced bysaid jet pump is converted to pressure head, said combustion chamberhaving a restricted entry connected to said recovery passage, said entrybeing of substantially smaller cross-section than said passage and thanthe burner chamber adjacent thereto, said entry adapted to produce aninlet velocity greater than the flame velocity of said gas, and a bluffbody flame holder means at said entry, said flame holder means in thepath of air fuel mixture entering said combustion chamber, producingeddy effects for stabilizing the combustion process, known per se, thatimprovement wherein said combustion chamber has an ignition recessadjacent said flame holder means and well upstream of the outlet of saidcombustion chamber, said ignition recess being out of the mainstream ofgases flowing from said flame holder means but in communicationtherewith and having spark means known per se for initially igniting theair fuel. mixture, and the said flame holder having insufficient eddyeffects to produce recirculation of gas from the outlet of saidcombustion chamber, but sufficient to produce recirculation of gas fromsaid ignition recess whereby ignition can be initiated while said flameholder introduces only a small pressure loss to gas entering saidcombustion chamber.

26. The burner according to claim 25 wherein said bluff body flameholder comprises a series of helical blades (known per se) the end ofsaid blades being tapered producing only a limited bluff body effect.

27. The burner of claim 25 wherein there are a plurality of parallelcombustion chambers, said ignition recess communicating directly orindirectly with each of said combustion chambers.

28. A hand held gun for directing a flow of heated air against a workobject, relying upon fuel alone without assistance of blowers orcompressors, said gun comprising the combination of a gaseous fuel jetadapted for connection to a conventional fuel gas source,

a jet pump activated by said gas jet and having an opening for drawingatmospheric air for combustion into a subatmospheric pressure regionproduced by said jet, said jet pump constructed to impart velocity tosaid combustion air by mixing,

an enlarged pressure recovery passage into which the mixture of gaseousfuel and combustion air proceeds, said recovery passage constructed toconvert velocity head of said gases to a pressure head exceedingatmospheric pressure, and

an internal combustion chamber arranged to receive for combustion saidgases discharging from said pressure recovery passage, said chamberhaving walls arranged to cooperate with said pressure head to converycombustionenhancing said mixing, and reducing the necesl sary length ofsaid entrainment zone by comparison with a similar gun having a circularcombustion chamber outlet.

29. A hand held gun for directing a flow of heated air against a workobject, relying upon fuel alone without assistance of blowers orcompressors, said gun comprising the combination of aa gaseous fuel jetadapted for connection to a conventional fuel gas source,

a jet pump activated by said gas jet and having an opening for drawingatmospheric air for combustion into a subatmospheric pressure regionproduced by said jet, said jet pump constructed to impart velocity tosaid combustion air by mixing,

an enlarged pressure recovery passage into which the mixture of gaseousfuel and combustion air proceeds, said recovery passage constructed toconvert velocity head of said gases to a pressure head exceedingatmospheric pressure, and

an internal combustion chamber arranged to receive for combustion saidgases discharging from said pressure recovery passage,

said chamber having walls arranged to cooperate with said pressure headto convert combustionrelated gaseous expansion into velocity head of thecombustion products,

said chamber having an outlet constructed and arranged to discharge saidcombustion products into an air entrainment zone for mixing saidproducts with, and transferring heat to, ambient air to produce at thedownstream end of said zone a flow of heated gases at a selectedtemperature,

said outlet being constructed and arranged to cause said combustionproducts to discharge into said zone in divergent outward directions,thereby providing an extending interface between said dischargingproducts and ambient air, enhancing said mixing, and reducing thenecessary length of said entrainment zone by comparison with a similargun having a combustion chamber arranged for straight-ahead discharge.

30. the gun of claim 29 wherein said combustion chamber has at least oneoutwardly flaring wall at said outlet.

12 31. The gun of claim 30 wherein said outlet is an elongated slit andsaid combustion chamber has outwardly flaring walls extending along themajor dimension of said slit.

32. The gun of claim 29 wherein said combustion chamber has a constantor decreasing flow cross-sectional area leading to said outlet.

33. A hand held gun for directing a flow of heated air against a workobject, relying upon fuel alone without assistance of blowers orcompressors, said gun comprising the combination of a gaseous fuel jetadapted for connection to a conventional fuel gas source, a jet. pumpadtivated by said gas jet and having an opening for drawing atmosphericair for combustion into a subatmospheric pressure region produced bysaid jet, said jet pump constructed to impart velocity to saidcombustion air by mixing, an enlarged pressure recovery passage intowhich the mixture of gaseous fuel and combustion air proceeds, saidrecovery passage constructed to convert velocity head of said gases to apressure head exceeding atmospheric pressure, an internal combustionchamber arranged to receive for combustion said gases discharging fromsaid pressure recovery passage, said chamber having walls arranged tocooperate with said pressure head to convert combustionrelated gaseousexpansion into velocity head of the combustion products,

said chamber having an outlet constructed and arranged to discharge saidcombustion products into an air entrainment zone for mixing saidproducts with, and transferring heat to, ambient air to produce at thedownstream end of said zone a flow of heated gases at a selectedtemperature, and

a temperature limiting structure extending downstream from saidcombustion chamber along said entrainment zone to prevent direct accessof the work object to said combustion chamber outlet.

34. The gun of claim 28 wherein said pressure recovery passage is partof an elongated passage which is effectively closed to the atmospherebetween said jet pump and said outlet.

35. The gun of claim 33 wherein said temperature limiting structure is atubular member having an air inlet flow cross section in the vicinity ofsaid outlet which is greater than five times the flow area of saidoutlet.

36. The gun of claim 35 wherein said tubular member has imperforatewalls extending along its length and has an effective flow capacity suchthat the ratio of said flow capacity to that of said outlet determinesthe temperature of the resultant flow at the downstream end of saidzone.

37. The gun of claim 36 wherein said tubular member has an outlet in theform of an elongated slit.

UNITED STATES PATENT AND TRADEMARK OFFICE QETTFMIATE 0F QEQTIGN Page 1of 2 PATENT NO. 2,9173%42 DATED November l, 1975 |NVENTOR(S) Dimiter SZagoroff St is certified that erro. appears in the above-identifiedpatent and that said Letters Patent hereby conecied as shown below:

Col 1, line 11, "about" should be -to-.

Col 2, lines 10 and. 11, "ration" should be --ratio-;

line 18, "momenntum" should be --momentum;

line 48, "present" should be -preset'.

Col. 2, line 36, "pumpting" should be -pumping;-

Col. 4, line 7, "verical" should be --vertical--; line 1?, "bay" shouldbe bag-;

line 66, "In the diffuser 3c" should not be in italicsu F C010 6, line5, "passageway" should be --passage--;

line 17, after "jet pump," insert --to pump;

line 22, "flare" should be -flared-;

line '52, "ration" should be --ratio--;

line 46, "branches" should be --branched--.,

UNITED STATES PATENT AND TRADEMARK OFFICE QETIQATE 0F COEC'HGN Page 2 of2 PATENT NO. ,917, r r2 DATED November t, 1975 |NVENTOR(S) Dimiter' S.Zagoroff It is certified that error appears in the ab0ve-identitiedpatent and that said Letters Patent are hereby corrected as shown below:9

Cole 7, line 15, after- "gun" insert --for-.

Q Col, 8, line 30, "vincity" should be --vicinity--.

Col. 9, line 52, "coon-" should be --con- Col, 10, line 68, "conver'y"should be --convert-.

0 Col. 11, line 23, "aa" should be -a--;

line 52, "extending" should be --extended-.

Signed an eaied this G first Day f June1976 {SEAL} Arrest:

RUTH C. MASON C. MARSHALL DANN AIR-""18 0.07" Commissioner oj'Parems andTrademarks

1. A hand held aimable shrink gun plastic film capable of providing ashrink-producing flow of heated air in the 250*F to 1000*F range againstplastic film lying over an object to be covered, the shrink gun relyingupon fuel along without assistance of blowers or compressors, saidshrink gun comprising a gas jet adapted for connection to a conventionalfuel gas source such as propane having a stoichiometric burningtemperature substantially exceeding 3000*F, a jet pump activated by saidgas jet and having an opening for drawing atmospheric air for combustioninto a subatmospheric pressure region produced by said jet, said jetpump constructed to impart velocity to said combustion air by mixing, anenlarged pressure recovery passage into which the mixture of gaseousfuel and combustion air proceeds, said recovery passage constructed toconvert velocity head of said gases to a pressure head exceedingatmospheric pressure, an internal combustion chamber having an entryinto which said pressure recovery passage discharges, said internalcombustion chamber having a flame holding means at said entry and anoutlet discharging into an ambient air propelling and mixing zonepreceding said work object, the respective parts of said shrink gunconstructed to introduce and burn said fuel in substantiallystoichiometric conditions and discharge combustion gases into saidpropelling zone at a temperature exceeding 3000*F and a velocity inexcess of 4000 feet per minute in a manner to propel relatively largerquantities of ambient air in the same direction with attendant heatingthereof by said combustion gases, thereby to produce a flow against saidplastic film at temperature in the 250*F to 1000*F range, consisting inmajor part of ambient air propelled and heated by said combustion gases.2. The shrink gun of claim 1 including positioning means setting aminimum length of said propelling and mixing zone between said workpiece and combustion chamber outlet, ensuring a flow of heated air belowa predetermined maximum temperature upon the work piece.
 3. the shrinkgun of claim 2 wherein the combustion gases are exposed during theirtravel through said propelling and mixing zone to admission ofincreasing quantities of air.
 4. The shrink gun of claim 2 wherein saidpositioning means comprises a shield member extending at least partlyabout the stream of combustion gases, said member providing air flowspace whereby ambient air can flow from surroundings into contact withsaid combustion gases.
 5. The shrink gun of claim 4 wherein said shieldmember has openings along its length exposing the traveling gases toadmission of additional air therealong.
 6. The shrink gun of claim 1wherein said combustion chamber outlet has a cross-section perimetergreater by at least 25% than the perimeter of a single circle ofidentical cross-sectional area.
 7. The shrink gun of claim 6 whereinsaid combustion chamber has a constant or decreasing flow cross-sectionarea leading to said outlet.
 8. The shrink gun of claim 7 wherein saidoutlet comprises an elongated outlet aperture, said combustion chamberhaving walls diverging in the direction of elongation of said aperture.9. The shrink gun of claim 6 wherein the outlet cross-section comprisesa multiplicity of elongated slits.
 10. The shrink gun of claim 1 whereinsaid outlet has portions directed in divergent outward directions. 11.The shRink gun of claim 10 wherein said outlet comprises a multiplicityof outlet apertures, axes of some of said apertures diverging relativeto other of said apertures.
 12. The shrink gun of claim 1 wherein saidcombustion chamber has a restricted entry connected to said recoverypassage, said entry being of substantially smaller cross-section thansaid passage and than the combustion chamber downstream thereof, saidentry adapted to produce an inlet velocity to said combustion chambergreater than the flame velocity of said gas.
 13. The shrink gun of claim12 including a tubular member surroundinng the outlet of said combustionchamber, said tubular member providing an air inlet flow cross-sectionin the vincity of said outlet which is greater than 5 times the flowarea of said combustion chamber outlet, said tubular member having alength longer than said combustion chamber.
 14. The shrink gun of claim13 wherein said tubular member has air inlet openings along its length,for adding air to the stream flowing through said tubular member. 15.The shrink gun of claim 13 constructed to produce a predetermined outlettemperature wherein walls along the length of said tubular member areimperforate, the inlet for ambient air to said tubular member positionedin the vicinity of said combustion chamber outlet, and being in therange of 5 to 50 times the volume of combustion gases, said outlet andsaid tubular member cooperating to provide a jet pump for ambient air.16. The shrink gun of claim 1 wherein said combustion chamber has anexhaust outlet flow cross-section of extended perimeter, preferably theoutlet formed as an elongated slit, said outlet defining a propellingmeans having an extended jet-air interface effective to propel and mixwith large quantities of ambient air over a short distance beforereaching said object to be heated.
 17. The shrink gun of claim 16wherein said combustion chamber in the region immediately preceding saidoutlet has walls arranged so that the flow cross-section does notincrease leading to said outlet, said walls cooperating to achieve ahigh exit velocity at said outlet for propelling said ambient air. 18.The shrink gun of claim 17 wherein said flow cross-section of saidcombustion chamber decreases leading toward said outlet while one pairof walls diverge from each other in the direction of said outlet toproduce a high velocity, divergent jet having a progressively enlargingjet-air pumping interface, for increased entrainment of air.
 19. Theshrink gun of claim 16 wherein there are a plurality of outlet sectionsfor said high velocity gas, preferably these outlet sections being setat angles to one another, providing a large jet-air pumping interfacefor increased entrainment of air.
 20. The shrink gun according to claim19 wherein a single combustion chamber has angled walls defininggradually diverging streams, the ends of the walls defining saidplurality of combustion chamber outlet sections.
 21. The shrink gun ofclaim 19 wherein certain outlet sections are associated with differentcombustion chambers, all of said combustion chambers receiving fuel-airmixture from a common mixing chamber comprising a primary jet pumpactivated only by a jet of gaseous fuel.
 22. The shrink gun of claim 1having a bluff body flame holder means in the path of air-fuel mixtureentering said combustion chamber, producing eddy effects for stabilizingthe combustion process, known per se, and characterized in that saidcombustion chamber has an ignition recess adjacent said flame holdermeans and well upstream of the outlet of said combustion chamber, saidignition recess being out of the mainstream of gases flowing from saidflame holder means but in communication therewith and having spark meansknown per se for initially igniting the air fuel mixture, and the saidflame holder having insufficient eddy effects to produce recirculationof gas from the outlet of said combustion chamber, but sufficieNt toproduce recirculation of gas from said ignition recess whereby ignitioncan be initated while said flame holder introduces only a small pressureloss to gas entering said combustion chamber.
 23. The shrink gunaccording to claim 22 characterized in that said bluff body flame holdercomprises a series of helical blades (known per se), the end of saidblades being tapered producing only a limited bluff body effect.
 24. Ahand held aimable shrink gun for plastic film capable of providing ashrink-producing flow of heated air in the 250*F to 1000*F range againstplastic film lying over an object to be covered, the shrink gun relyingupon fuel alone without assistance of blowers or compressors, saidshrink gun comprising a gas jet adapated for connection to aconventional fuel gas source such as propane having a stoichiometricburning temperature substantially exceeding 3000*F, a jet pump activatedby said gas jet and having an opening for drawing atmospheric air forcombustion into a subatmospheric pressure region produced by said jet,said jet pump constructed to impart velocity to said combustion air bymixing, an enlarged pressure recovery passage into which the mixture ofgaseous fuel and combustion air proceeds, said recovery passagecoonstructed to convert velocity head of said gases to a pressure headexceeding atmospheric pressure, a plurality of internal combustionchambers each having an entry into which said pressure recovery passagedischarges, each of said internal combustion chambers having a flameholding means at its said entry and an outlet discharging into anambient air propelling and mixing zone preceding said work object, therespective parts of said shrink gun constructed to introduce and burnsaid fuel in substantially stoichiometric conditions and dischargecombustion gases into said propelling zone at a temperature exceeding3000*F and a velocity in excess of 4000 feet per minute in a manner topropel relatively larger quantities of ambient air in the same directionwith attendant heating thereof by said combustion gases, the outlets ofsaid plurality of combustion chambers combining to produce a flowagainst said plastic film at temperature in the 250*F to 1000*F range,consisting in major part of ambient air propelled and heated by saidcombustion gases.
 25. In a burner for gaseous fuel activated only by theambient pressure of a liquid gas fuel source, suitable for use in heatguns and torches, and comprising a mixing chamber directing air fuelmixture into a combustion chamber, the mixing chamber in the form of aprimary jet pump activated only by a jet of fuel from said source whichjet entrains air from the atmosphere and forms a fuel-air mixture, apressure recovery passage of increased cross-section wherein velocityhead produced by said jet pump is converted to pressure head, saidcombustion chamber having a restricted entry connected to said recoverypassage, said entry being of substantially smaller cross-section thansaid passage and than the burner chamber adjacent thereto, said entryadapted to produce an inlet velocity greater than the flame velocity ofsaid gas, and a bluff body flame holder means at said entry, said flameholder means in the path of air fuel mixture entering said combustionchamber, producing eddy effects for stabilizing the combustion process,known per se, that improvement wherein said combustion chamber has anignition recess adjacent said flame holder means and well upstream ofthe outlet of said combustion chamber, said ignition recess being out ofthe mainstream of gases flowing from said flame holder means but incommunication therewith and having spark means known per se forinitially igniting the air fuel mixture, and the said flame holderhaving insufficient eddy effects to produce recirculation of gas fromthe outlet of said combustion chamber, but sufficient to producerecirculation of gas from said ignition recess whereby ignitioN can beinitiated while said flame holder introduces only a small pressure lossto gas entering said combustion chamber.
 26. The burner according toclaim 25 wherein said bluff body flame holder comprises a series ofhelical blades (known per se) the end of said blades being taperedproducing only a limited bluff body effect.
 27. The burner of claim 25wherein there are a plurality of parallel combustion chambers, saidignition recess communicating directly or indirectly with each of saidcombustion chambers.
 28. A hand held gun for directing a flow of heatedair against a work object, relying upon fuel alone without assistance ofblowers or compressors, said gun comprising the combination of a gaseousfuel jet adapted for connection to a conventional fuel gas source, a jetpump activated by said gas jet and having an opening for drawingatmospheric air for combustion into a subatmospheric pressure regionproduced by said jet, said jet pump constructed to impart velocity tosaid combustion air by mixing, an enlarged pressure recovery passageinto which the mixture of gaseous fuel and combustion air proceeds, saidrecovery passage constructed to convert velocity head of said gases to apressure head exceeding atmospheric pressure, and an internal combustionchamber arranged to receive for combustion said gases discharging fromsaid pressure recovery passage, said chamber having walls arranged tocooperate with said pressure head to convery combustion-related gaseousexpansion into velocity head of the combustion products, said chamberhaving an outlet constructed and arranged to discharge said combustionproducts into an air entrainment zone for mixing said products with, andtransferring heat to, ambient air to produce at the downstream end ofsaid zone a flow of heated gases at a selected temperature, said outlethaving an effective wetted perimeter at least 25% longer than that of asingle circle of identical cross-sectional area, thereby providing anextended interface between combustion products discharging from saidoutlet and ambient air, enhancing said mixing, and reducing thenecessary length of said entrainment zone by comparison with a similargun having a circular combustion chamber outlet.
 29. A hand held gun fordirecting a flow of heated air against a work object, relying upon fuelalone without assistance of blowers or compressors, said gun comprisingthe combination of aa gaseous fuel jet adapted for connection to aconventional fuel gas source, a jet pump activated by said gas jet andhaving an opening for drawing atmospheric air for combustion into asubatmospheric pressure region produced by said jet, said jet pumpconstructed to impart velocity to said combustion air by mixing, anenlarged pressure recovery passage into which the mixture of gaseousfuel and combustion air proceeds, said recovery passage constructed toconvert velocity head of said gases to a pressure head exceedingatmospheric pressure, and an internal combustion chamber arranged toreceive for combustion said gases discharging from said pressurerecovery passage, said chamber having walls arranged to cooperate withsaid pressure head to convert combustion-related gaseous expansion intovelocity head of the combustion products, said chamber having an outletconstructed and arranged to discharge said combustion products into anair entrainment zone for mixing said products with, and transferringheat to, ambient air to produce at the downstream end of said zone aflow of heated gases at a selected temperature, said outlet beingconstructed and arranged to cause said combustion products to dischargeinto said zone in divergent outward directions, thereby providing anextending interface between said discharging products and ambient air,enhancing said mixing, and reducing the necessary length of saidentrainment zone by comparison with a similar gun having a combustionchamber arranged for straigHt-ahead discharge.
 30. the gun of claim 29wherein said combustion chamber has at least one outwardly flaring wallat said outlet.
 31. The gun of claim 30 wherein said outlet is anelongated slit and said combustion chamber has outwardly flaring wallsextending along the major dimension of said slit.
 32. The gun of claim29 wherein said combustion chamber has a constant or decreasing flowcross-sectional area leading to said outlet.
 33. A hand held gun fordirecting a flow of heated air against a work object, relying upon fuelalone without assistance of blowers or compressors, said gun comprisingthe combination of a gaseous fuel jet adapted for connection to aconventional fuel gas source, a jet pump adtivated by said gas jet andhaving an opening for drawing atmospheric air for combustion into asubatmospheric pressure region produced by said jet, said jet pumpconstructed to impart velocity to said combustion air by mixing, anenlarged pressure recovery passage into which the mixture of gaseousfuel and combustion air proceeds, said recovery passage constructed toconvert velocity head of said gases to a pressure head exceedingatmospheric pressure, an internal combustion chamber arranged to receivefor combustion said gases discharging from said pressure recoverypassage, said chamber having walls arranged to cooperate with saidpressure head to convert combustion-related gaseous expansion intovelocity head of the combustion products, said chamber having an outletconstructed and arranged to discharge said combustion products into anair entrainment zone for mixing said products with, and transferringheat to, ambient air to produce at the downstream end of said zone aflow of heated gases at a selected temperature, and a temperaturelimiting structure extending downstream from said combustion chamberalong said entrainment zone to prevent direct access of the work objectto said combustion chamber outlet.
 34. The gun of claim 28 wherein saidpressure recovery passage is part of an elongated passage which iseffectively closed to the atmosphere between said jet pump and saidoutlet.
 35. The gun of claim 33 wherein said temperature limitingstructure is a tubular member having an air inlet flow cross section inthe vicinity of said outlet which is greater than five times the flowarea of said outlet.
 36. The gun of claim 35 wherein said tubular memberhas imperforate walls extending along its length and has an effectiveflow capacity such that the ratio of said flow capacity to that of saidoutlet determines the temperature of the resultant flow at thedownstream end of said zone.
 37. The gun of claim 36 wherein saidtubular member has an outlet in the form of an elongated slit.